cprop: Fix -fcompare-debug bug in constprop_register [PR100148]

cprop: Fix -fcompare-debug bug in constprop_register [PR100148]

The following testcase shows different behavior between -g and -g0
in constprop_register, if a flags register setter is separated
from a conditional jump using those flags with -g by a DEBUG_INSN.
As it uses just NEXT_INSN, for -g it will look at the DEBUG_INSN which is
not a conditional jump, while otherwise it would look at the conditional
jump and call cprop_jump.

2021-04-21  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/100148
	* cprop.c (constprop_register): Use next_nondebug_insn instead of
	NEXT_INSN.

	* g++.dg/opt/pr100148.C: New test.